The pneumatic tire has proven its worth in providing a comfortable ride with load carrying capabilities for automobiles, trucks, aircraft, and other vehicles. However, the tire carcass is susceptible to punctures which causes the tire to go flat rendering it unusable. A tire suddenly going flat can be life threatening as well as inconvenient and cause financial loss in industrial applications.
Very soft polyurethane elastomer-filled, deflation-proof pneumatic tires were developed beginning in the 1970's to reduce the downtime caused by flats in many industrial applications. A few of the prior art references in this field include: Gomberg, U.S. Pat. No. Reissue 29,890; Wyman, U.S. Pat. No. 4,416,844; Wyman, U.S. Pat. No. 4,683,929; Ford, U.S. Pat. No. 4,094,353; Kaneda, et al., U.S. Pat. No. 4,230,168; Bulluck, U.S. Pat. No. 5,070,138; and Gupta, U.S. Pat. No. 5,402,839. The Gupta patent is directed to polyurea-containing polyurethane elastomer filling materials having at least 1.00 weight percent aromatic polyamines.
Although the polyurethane filling materials give a harder ride than air, and add, in some cases, tremendous weight to the vehicle, the availability of deflation-proof tires is economically practical for many applications, such as, mining, scrap yards, military, and heavy construction.
Since air is negligible in weight and essentially free, the raw materials used in the compositions to fill the tires and to create the extra weight have to be inexpensive to make deflation-proof tires practical. Usually a two component system is meter-mixed together at a 1 to 1 ratio with basic pumping equipment through a static mixer as it is pumped into the tire to cure the mixture to a soft elastomer. Although polyurethane filling systems can be more expensive than other rubber materials, they can be formulated easily to a 1 to 1 ratio, can be made inexpensive by blending with high levels of plasticizing process oils and can have good performance properties.
Because many industrial applications require the filled tires to withstand high load carrying without large tire deflection and minimum heat buildup in the tire, the elastomer in the tire has to be very resilient and have a very low hysteresis or very low internal friction properties. Under extreme conditions, the core of the filled tire should not reach temperatures greater than 300.degree. F. and maintain an equilibrium temperature of less than 300.degree. F. The drawbacks of the current commercially available polyurethane filling compositions are they have only adequate resilience and hysteresis.
Most of the tire filling systems in the world today use toluene diisocyanate (TDI) because it is inexpensive, liquid, and relatively easy to work with. However, TDI has a high vapor pressure and like all isocyanates has a threshold limit for toxicity. If methylene diphenylisocyanate (MDI) could be used, it would be the preferred isocyanate because it has a much lower vapor pressure than TDI and is subjected to less environmental restrictions. While it is true a number of prior art references disclose and claim the use of MDI in tire filling compositions, such compositions have generally not had good enough performance to be commercially acceptable. Therefore, even though MDI is preferred from an environmental standpoint, few commercially acceptable polyurethane-filled pneumatic tires contain MDI. For example, some of the MDI-containing formulations that failed road testing had excellent tensile-tear properties, but the resilience or vertical rebound as measured by the ASTM D 2632-79 test, also known as the Bashore Rebound test, was inferior, i.e., resilience in the range of 46-53%. In general, the road test consisted of placing the filled tires on a pickup truck with a given load and driving the truck at a given range of speed for a finite distance. More definitely, the standardized simulated road test FMVSS (Federal Motor Vehicle Safety Standard) #119 Durability Test can be used. The specific conditions for such a road test are set forth under the Description of the Preferred Embodiments of the Invention. All of the tires filled with a prior art filling composition failed as a result of an increase of temperature indicating abnormal heat buildup because of the higher than normal hysteresis properties of the filling compositions.
Kaneda et al., U.S. Pat. No. 4,230,168 discloses and claims tire filling elastomers of polyoxypropylene polyols having an OH equivalent weight of 900 to 1800 and a functionality of 2 to 4, a polyisocyanate including MDI, and a chlorinated paraffin or dialkyl phthalate plasticizer miscible with the polyol. Such elastomeric filling materials are reported to have high resilience, as measured by a Dunlop-Resilience Tester, and low JIS hardness. This reference teaches that when attempts were made to improve the riding comfort of the filled tire by lowering the hardness of the filling material using a method that decreases the equivalent ratio of isocyanate groups to hydroxyl groups in the composition, the resilience is disadvantageously lowered. The Kaneda et al. reference also teaches against going outside the claimed range of OH equivalent weight for the polyols because the resilience of the tire filler is poor if the OH equivalent weight is below 900 or above 1800.
Toluene diisocyanate (TDI) has been the isocyanate of choice used in the polyurethane elastomer compositions not only for the foregoing reasons but it also has the advantage of more favorable economics when compared with MDI compositions. The polyurethane elastomers, which are currently commercially available for tire deflation-proofing applications containing TDI, have performed well under most conditions. However, when extreme loading is necessary, these systems will develop high internal heat and degrade causing tire failure. The resiliency of these systems is about 50% by the Bashore Rebound test. If resiliency could be improved, there would be greater stability of the tire filling systems under higher stress because there would be less heat created in the tires.
There is a great need for an MDI-containing tire filling formulation that has adequate tensile-tear properties and low hysteresis, without the tendency for excessive heat buildup. There is also a need for a relatively inexpensive MDI-containing tire filling formulation that has higher resilience over a wide range of hardness than current commercially available formulations.